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New Study Reveals Why "Cock-Eyed Squid" Has Two Different Sized Eyes

Marine biologists have wondered for years why so-called "cock-eyed squids" (Histioteuthis heteropsis) one huge eye and another smaller eye on their heads. Today, thanks to the great work of a few scientists at Duke University, the marine biology community has a greater understanding of this odd natural phenomenon.

Kate Thomas, a professor of marine biology at Duke University, recently led a study looking into over 150 videos of cock-eyed squids from the Monterey Bay area of California. Submarines captured all of these videos over 26-year span of time.

Thomas found that all of the cock-eyed squids' eyes actually looked in two opposing directions. The larger eye always looked upward, while the smaller eye looked downwards. Although many in the marine biology field proposed this theory in the past, Dr. Thomas was the first to actually show how this works with tangible data.

The cock-eyed squid, which is officially a part of the histioteuthid family, uses the larger eye to look for any prey it might be able to catch silhouetted against the natural sunlight. The smaller eye, however, searches for bioluminescent organisms in darker layers of the sea. Typically, the cock-eyed squid is born with relatively normal-sized eyes, but the left eye soon begins rapidly bulging during juvenile development.

All of the videos Dr. Thomas studied showed the cock-eyed squid swimming with its tail facing upwards. The squid's larger eye always looked up towards the sky to track prey it might see outlined against the light of the sun.

When asked why the two eyes were so different in terms of size, Dr. Thomas told reporters that a larger upward facing eye greatly enhances the squid's overall visual perception. On the other hand, it makes no sense evolutionarily for the downward facing eye to bulge out, since it's tasked with making out bioluminescent images in darker regions of the ocean.

Researchers said that the cock-eyed squid tends to have a great deal of yellow pigmentation in the lens of the larger eye. They believe this pigment might help the squid tell the difference between the bioluminescence of certain prey (e.g. lanternfish) and the natural sunlight.

Although the size difference between the cock-eyed squid is startling, it's not the only squid in the histioteuthid family to have developed two very different eyes. Indeed, Professor Thomas notes that many other squids have a slightly larger upward facing eye with numerous smaller eyes on the sides. The only major difference with the cock-eyed squid and other histioteuthid squids is the massive size difference between the two eyes.

Most cock-eyed squids are found in the ocean between a depth of 200 and 1,000 meters. Scientists first discovered these squids around 100 years ago, and they have been fascinated with them ever since.

The Royal Society Publishing has published Dr. Thomas's study in the journal Philosophical Transactions of the Royal Society B. This study was published under the title "Two eyes for two purposes: in situ evidence for asymmetric vision in the cockeyed squids Histioteuthis heteropsis and Stigmatoteuthis dofleini."